This invention relates to an image signal processing circuit for optically reading documents, pictures, etc., converting the thus detected optical signal into an electrical signal, and then processing the same to obtain a binary signal representing white (a background portion) or black (information such as characters or pictures) color.
FIG. 1(A) shows a typical example of an electric signal obtained by optically reading an original such as a document or picture. In the drawing, the solid line waveform represents a read-out signal for one scanning line including a resolution pattern section A and a document/picture section B. Also in the figure, BL denotes a black level and WL denotes a white level.
When obtaining white/black binary signals from such a read-out signal, heretofore a fixed threshold level SL was preset, and the white/black binary signals as illustrated in FIG. 1(B) were output in response to whether the read-out signal was higher or lower than a certain voltage (or current) level corresponding to the threshold level SL.
However, where the fixed threshold level SL is used, if image density is increased as shown in the resolution pattern section A, for example, the difference between the white and black level is reduced and the read-out signal has a tendency to rise up toward the white level WL as a whole, as is apparent from FIG. 1(A), thus resulting in a fear that it may become impossible to resolve the image through processing into the white/black binary signals, even if white and black colors can be correctly discriminated on the read-out image signal (or the image can be resolved optically).
Moreover, as is seen from a section C of the document/picture section B of FIG. 1(A), when reading a narrow white line on a black background, the read-out signal approaches the black level BL, resulting in that the narrow white line cannot be resolved.
As a result, in the conventional binary circuit using the fixed threshold value, there is a large difference in the amount of information in the input image signal as opposed to the output image signal.
FIG. 1(B) illustrates binary signals which were obtained by processing the read-out signal of FIG. 1(A) by use of the fixed threshold SL. It is seen from FIG. 1(B) that some information was lost in both sections A and C, thus resulting in poor resolution.
FIG. 2(A) illustrates a read-out signal which was obtained by sampling outputs from a one-dimensional image sensor such as a CCD along the scanning direction, and digitizing the sampled output. When the read-out signal is processed to form binary signals by use of the fixed threshold level SL, there are obtained binary signals as shown in FIG. 2(B). Hatched portions in FIG. 2(B) indicate the fact that the level of the read-out signal fluctuates in the vicinity of the threshold value, and hence the binary signals come to have a white-and-black "striped" form. It will be understood that a black portion is narrowed in section A, and a white portion is lost in section B, in the processillustrated in FIGS. 2(A) and (B).